Texas Instruments CDCM7005 manual Programming Interfaces J30, J31, Loop Filter J32−J34

Hardware Configuration

3.2.3Programming Interfaces (J30, J31)

The SPI of the device is used for writing to the control register of the device. It consists of three control lines CTRL_CLK, CTRL_DATA, and CTRL_LE. There are four 30-bit wide RAM registers, which can be addressed by the two LSBs of a transferred word. Every transmitted word must have 32 bits, starting with MSB. After supplying power or activating the power-down mode, the registers are loaded with the device default values internally (see the CDCM7005 data sheet, SCAS793). However, if specific register settings are required for any applications, there are two ways to program the device externally:

-Connect the parallel port cable to the PC and EVM parallel port. This needs control S/W (see Chapter 4).

3.2.4Loop Filter (J32−J34)

The loop filter is one of the key elements determining the loop bandwidth of the PLL. The loop filter converts the charge pump current into the control voltage for the voltage controlled oscillator. The phase difference between the input clocks of the phase frequency detector determines the width of the charge pump output current pulses. These high frequency pulses are transformed into a voltage to control the oscillator.

Basically, three types of loop filters are implemented on the EVM.

-Passive loop filter

-External active loop filter using an external low-noise OPA.

Filter types can be selected by soldering bridges J32−J34, see Table 3−1. Control voltage of the VC(X)O can be measured at J9 or TP1. If an external OPA is used, it needs to be switched on by connecting J34. For example, passive filter operation is provided when pads 1 and 3 of J33 are solder bridged and pads 1 and 3 of J32 are solder bridged.

Default setting: Passive Loop Filter

Table 3−1. Filter Configurations

Bridge	Passive Filter	Active With An External OPA
J33	1−3	1−2
J34	Open	Closed
J32	1−3	1−2

3.2.5High-Speed Outputs and Inputs (J1−J4, J6−J11, J13, J14, J22, and J23)

The CDCM7005 drives five differential LVPECL outputs. All PECL outputs are ac-coupled and terminated with 150 Ω to GND. This is in contrast to typical LVPECL termination, which requires VCC − 2 V as termination voltage. The reason is to simplify the power supply scheme. The device output’s trace impedance is 50 Ω and traces are matched in length. All outputs have options for pullup and pulldown resistors.

EVM Hardware 3-3